Method for producing sodium chloride crystals

ABSTRACT

Process for the production of sodium chloride crystals from a sodium chloride brine contaminated by potassium chloride and sulphate ions, according to which a calcium compound ( 32 ) is added to the brine ( 48 ) to crystallize glauberite ( 35 ), which is isolated, the resulting aqueous solution ( 36 ) is subjected to evaporation to crystallize sodium chloride ( 40 ), which is collected, and the aqueous mother liquor ( 41 ) from the crystallization of the sodium chloride is subjected to cooling ( 42 ) to crystallize glaserite ( 45 ).

The present invention relates to a process for the production of sodiumchloride crystals.

A widely used technique for producing solid sodium chloride consists incrystallizing it by evaporation of a sodium chloride brine. The sodiumchloride brine used in this technique can be, for example, seawater. Inan alternative form, it can be obtained by dissolution of rock salt inwater. The brines used in this technique for the manufacture of sodiumchloride are usually contaminated by undesirable compounds which riskcontaminating the sodium chloride. This is in particular the case withpotassium chloride, sodium carbonate and sodium sulphate which, if theyare not removed from the brine before evaporation of the latter, couldeasily be encountered in association with the sodium chloride crystals.The removal of these undesirable compounds from the brine moreoverpresents the problem of their discharge and of their storage (generallyin public landfill sites) under conditions suited to preventing themfrom polluting the environment.

A process for the manufacture of sodium chloride crystals starting froma sodium chloride brine contaminated by potassium chloride and sulphateions is known (U.S. Pat. No. 4,547,197). According to this knownprocess, calcium hydroxide is added to the brine, to precipitate thesulphate ions in the form of calcium sulphate, and then sodium carbonateis added, to precipitate the calcium ions in the form of calciumcarbonate. The aqueous solution collected after separation of theprecipitates is subjected to evaporation at a temperature ofapproximately 120° C. to crystallize sodium chloride, which iscollected, and the aqueous mother liquor from the crystallization iscooled to approximately 45° C. (preferably by subjecting it to areduction in pressure) to crystallize simultaneously sodium chloride,potassium chloride and sodium sulphate. In this known process, thecoprecipitation of the three salts (sodium chloride, potassium chlorideand sodium sulphate) causes difficulties in the discharge of the latter.A solution suggested in the document U.S. Pat. No. 4,547,197 consists indispersing the mixture of the three salts in cold water, so as toselectively dissolve the sodium chloride and the potassium chloride,while the sodium sulphate recrystallizes in the form of Glauber's salt.

The abovementioned known process exhibits a disadvantage of greatcomplexity, imposed by the need to dispose of the mixture of the threesalts (sodium chloride, potassium chloride and sodium sulphate) withoutharming the environment. This known process exhibits the additionaldisadvantage that it consumes a large amount of calcium compound(calcium hydroxide) in extracting the sulphate ions from the brine.

The invention is aimed at overcoming the disadvantages of the knownprocess described above by providing a process for the manufacture ofsodium chloride crystals from a brine contaminated by potassium chlorideand sulphate ions which makes possible a substantial reduction in theconsumption of calcium compound in extracting the sulphate ions from thebrine and which, moreover, simplifies thee discharge and the storage ofthe residual solid materials from the purification.

The invention consequently relates to a process for the production ofsodium chloride crystals from a sodium chloride brine contaminated bypotassium chloride and sulphate ions, according to which, in a firststage, a calcium compound is added to the brine to precipitate calciumsulphate, which is isolated, and an aqueous solution is collected, in asecond stage, the aqueous solution from the first stage is subjected toevaporation, to crystallize sodium chloride, and sodium chloridecrystals and an aqueous mother liquor are collected separately, and, ina third stage, the aqueous mother liquor from the second stage issubjected to cooling in order to crystallize at least a portion of thepotassium chloride; according to the invention, the amount of calciumcompound in the first stage and the cooling in the third stage areadjusted so that the sulphate ions precipitate in the form of glauberitein the first stage and of glaserite in the third stage.

In the process according to the invention, the sodium chloride brine is,by definition, an aqueous sodium chloride solution. It is invariably anaqueous solution which is saturated or unsaturated with sodium chloride.Its content by weight of sodium chloride is advantageously greater than5%, usually at least equal to 10%. Brines comprising at least 20% byweight of sodium chloride are especially recommended. Brines which aresubstantially saturated at ambient temperature are preferred.

The sodium chloride brine employed in the process according to theinvention is contaminated by impurities. These impurities comprisepotassium chloride and sulphate ions. The sulphate ions are, forexample, present in the form of dissolved sodium sulphate. Theimpurities are normally present in brine in an amount of less than thesodium chloride content.

In the first stage of the process according to the invention, the roleof the calcium compound is to react with the sulphate ions to formcalcium sulphate, which crystallizes. The calcium compound mustconsequently be chosen from those which are capable of reacting withsulphate ions, in particular with alkali metal sulphates (in particularsodium sulphate), to form calcium sulphate. The calcium compoundemployed in the first stage is advantageously a water-soluble compound.Calcium chloride is preferred.

On conclusion of the first stage, the calcium sulphate crystals areisolated from the brine. The means employed for this purpose is notcritical. It advantageously comprises a filtration or a sedimentationfollowed by a separation.

In the second stage of the process according to the invention, theaqueous solution collected after separation of the crystals from thefirst stage is subjected to evaporation in order to crystallize sodiumchloride. The parameters of the evaporation (in particular temperature,pressure and the degree of evaporation) are chosen so as to avoidsimultaneous crystallization of undesirable compounds, such as potassiumchloride or sodium sulphate. The optimum values of these parameters willdepend on the concentration of the brine, on its contents of potassiumchloride and of sulphate ions and, if appropriate, on the otherimpurities present. They can be easily determined by routine work, fromliquid-solid equilibrium diagrams, in particular the Na—K—Cl—SO₄—H₂Odiagram, accessible from the information in the literature.

On conclusion of the second stage, the sodium chloride crystals areseparated from the aqueous mother liquor. The said separation can becarried out by any appropriate means, for example by filtration, bycentrifuging or by sedimentation followed by separation.

In the third stage of the process according to the invention, theaqueous mother liquor from the second stage is subjected to controlledcooling in order to render insoluble and to crystallize the potassiumchloride.

In accordance with the invention, the calcium compound is employed inthe first stage in an amount which is in deficiency with respect to thatwhich is necessary to convert all the sulphate ions to calcium sulphate.More specifically, the amount of calcium compound employed in the firststage is adjusted according to the potassium content of the brine, sothat a fraction of the sulphate ions of the brine precipitates in theform of glauberite (mixed calcium sulphate and sodium sulphate compoundof general formula CaSO₄.Na₂SO₄) in the first stage and so that thebalance of the sulphate ions precipitates with all the potassium ions inthe form of glaserite (mixed potassium sulphate and sodium sulphatecompound of general formula Na₂SO₄.3K₂SO₄) in the third stage. Theremoval of the sulphate ions and potassium ions by successivecrystallizations of glauberite and glaserite is based on a sulphatecontent in the brine which is greater than that necessary to precipitateall the potassium ions in the form of glaserite. In practice, the brineshould consequently comprise a molar amount of sulphate ions of greaterthan 1.5 times its molar content of potassium ions. The parameters ofthe process in the first stage (the temperature and the amount ofcalcium compound employed), in the second stage (the temperature, thepressure and the degree of evaporation) and in the third stage (thetemperature and the pressure) should be determined in each specific caseaccording to the concentration of the brine and according to itsrespective contents of potassium chloride and of sulphate ions. Theoptimum values of these parameters can be easily determined bycalculation and from the liquid-solid equilibrium diagrams.

In practice, good results are obtained when the evaporation, in thesecond stage, is carried out at a temperature of greater than 75° C. andnot exceeding 200° C. (temperatures of 80 to 120° C. being preferred).The pressure should be adjusted to the temperature selected and can beless than standard atmospheric pressure in the case of low temperaturesor greater than the latter in the case of high temperatures. The coolingin the third stage is advantageously carried out at a temperature ofless than 80° C., preferably of 10 to 70° C., temperatures in the regionof ambient temperature (for example from 15 to 30° C.) beingrecommended.

It may happen that the aqueous solution collected from the first stagecomprises dissolved calcium ions. To this end, according to a specificembodiment of the process according to the invention, the aqueoussolution collected from the first stage has sodium carbonate added to itbefore carrying out the evaporation in the second stage. In thisembodiment of the process according to the invention, the role of thesodium carbonate is to react with the residual calcium ions tocrystallize calcium carbonate, which is removed by any appropriatemeans. The amount of sodium carbonate employed in this embodiment of theinvention can be easily determined from the residual amount of calciumions in the aqueous solution.

In addition to the potassium chloride and sulphate ions, the sodiumchloride brine subjected to the process according to the invention canoptionally comprise other dissolved impurities, in particular sodiumcarbonate. To this end, according to another embodiment of the processaccording to the invention, the brine is treated, upstream of the firststage, by any appropriate means for removing the sodium carbonate whichit comprises. To this end, in a first alternative implementation of thisembodiment of the process, the brine is treated, upstream of the firststage, with hydrochloric acid in order to decompose the sodium carbonateand to form sodium chloride. The carbon dioxide generated by thereaction is discharged from the brine by any appropriate degassingmeans. In a second alternative implementation, the brine is treated,upstream of the first stage, with carbon dioxide in order to crystallizesodium bicarbonate, which is collected.

The process according to the invention applies well to sodium chloridebrines which comprise, per kg of dry matter, from 550 to 800 g of sodiumchloride, from 50 to 350 g of sodium sulphate and from 5 to 100 g ofpotassium chloride, and optionally from 50 to 250 g of sodium carbonate.It finds an application in the treatment of seawater or sodium chloridebrines obtained by dissolution of rock salt. The invention is especiallysuited to waste brines from the purification treatments, by means ofbasic sodium compounds, such as sodium hydroxide or sodium bicarbonate,of flue gases contaminated by hydrogen chloride. The invention isespecially suited to the treatment of the brines produced by dispersing,in water, the residual material which is obtained after purification, bymeans of a basic reactant selected from sodium carbonate, sodiumbicarbonate and sodium sesquicarbonate, of a flue gas originating fromthe incineration of waste comprising chlorinated compounds (inparticular waste of domestic or hospital origin or some types ofindustrial waste) In this specific application of the process accordingto the invention, the brine is preferably subjected to a preliminarytreatment for purification from heavy metals, for example by applyingthe technique disclosed in the document EP-B-603 218 [Solvay (SociétéAnonyme)].

Distinctive features and details of the invention will emerge from thefollowing description of the appended drawings.

FIG. 1 represents the scheme of a specific embodiment of the processaccording to the invention.

FIG. 2 shows a detail of an alternative form of the embodiment of FIG.1.

In these figures, identical reference notations denote identicalcomponents.

In the embodiment in FIG. 1, the process according to the invention isapplied to a sodium chloride brine obtained from a residual materialfrom the purification of a flue gas generated by the incineration ofwaste comprising organic and inorganic chlorinated compounds.

It is known that domestic waste, hospital waste and some types of wasteof industrial origin comprise organic and inorganic chlorinatedcompounds, the incineration of which gives rise to the emission of fluegases contaminated by hydrogen chloride.

The plant in FIG. 1 comprises a furnace 1 which is fed with domestic orhospital waste 2. The flue gas 3 emitted by the furnace 1 iscontaminated by hydrogen chloride, volatile heavy metals and sulphurdioxide. It is furthermore laden with ash. It is first treated in adeduster 4 (for example a cyclone or an electrostatic filter) in orderto separate the ash 5 therefrom. The dedusted flue gas 6 collected fromthe deduster 4 is introduced into the reaction chamber 7 where sodiumbicarbonate 8 is added to it in the form of a powder. The sodiumbicarbonate is introduced into the flue gas in an amount sufficient todecompose all the hydrogen chloride and sulphur dioxide in the flue gasand to form sodium chloride and sodium sulphate. The flue gas 9collected from the reaction chamber 8 is treated on a filter 10 todedust it and the thus dedusted flue gas 11 is discharged at the chimney12. The filter 10 preferably comprises a filter with a filter cloth (forexample a sleeve filter).

The dust 13 retained in the filter 10 comprises sodium chloride, sodiumsulphate, potassium chloride, heavy metals and sodium carbonateoriginating from the excess sodium bicarbonate employed. It is conveyedto a dispersion chamber 14 where it is dispersed in water 15, so as todissolve the sodium chloride and the other water-soluble compounds inthe dust. Use is advantageously made of water which has been used towash the ash 5. The aqueous medium 16 collected from the dissolutionchamber 14 comprises an aqueous sodium chloride solution contaminated bydissolved impurities which comprise in particular heavy metals,potassium chloride, sodium sulphate and sodium carbonate. The aqueousmedium 16 is conveyed to a reaction chamber 17 where a sufficient amountof sodium hydroxide 18 is added to it to precipitate the heavy metals inthe form of hydroxides. The aqueous suspension 19 collected from thereaction chamber 17 is conveyed to a filter 20 where a precipitate 21,comprising heavy metal hydroxides and, if appropriate, various otherresidual insoluble compounds, is separated. The filtrate 22 is conveyedto a column 23 where it moves in contact with a chelating resin forremoving the final traces of heavy metals. The sodium chloride brine 24is collected from the column 23. This brine is substantially saturatedwith sodium chloride and it is contaminated by potassium chloride,sodium sulphate and sodium carbonate. It comprises, for example, per kgof dry matter, from 700 to 750 g of sodium chloride, from 10 to 50 g ofpotassium chloride, approximately 100 g of sodium sulphate andapproximately 100 g of sodium carbonate.

In accordance with the invention, the brine 24 is introduced into areaction chamber 25 where it is treated with an aqueous hydrochloricacid solution 47 in an amount sufficient to decompose the sodiumcarbonate and to form sodium chloride. The brine 28 collected from thereaction chamber 25 is subjected to flushing with a stream of air 30 ina column 29 to discharge the carbon dioxide 49 generated by thedecomposition of the sodium carbonate. In a reaction chamber 31 situateddownstream of the column 30, calcium chloride 32 is added to thedegassed brine 48 exiting from the column 29 to precipitate a portion ofthe sulphate ions in the form of glauberite (double calcium and sodiumsulphate). The calcium chloride 32 is, however, employed in an amountinsufficient to react with all the sulphate ions in the brine 48. Theaqueous suspension 33 collected from the chamber 31 is treated on afilter 34 where the glauberite precipitate 35 and an aqueous solution 36are separated, the glauberite precipitate being discharged. The aqueoussolution is conveyed to an evaporator-crystallizer 37 where it issubjected to partial evaporation.

The pressure, the temperature and the degree of evaporation in theevaporator-crystallizer 37 are chosen to crystallize sodium chloridewhile expressly avoiding the crystallization of; other compounds, inparticular potassium chloride and sodium sulphate. The evaporation can,for example, be carried out at a temperature of 80 to 120° C. A slurryof crystals 38 is withdrawn from the crystallizer 37, from which slurrysodium chloride crystals 40 of high purity are collected on a filter 39,which crystals are exploited as is, for example in an industrialprocess.

The aqueous mother liquor 41 from the crystallizer 37 comprisespotassium chloride and sodium sulphate and it cannot be discharged as isto the environment. It is treated in the crystallizer 42 where it iscooled to a sufficiently low temperature (for example to the ambienttemperature of 20 to 25° C.) to crystallize glaserite (double potassiumand sodium sulphate). A slurry of crystals 43 is withdrawn from thecrystallizer 42, which slurry is subjected to filtration 44. Thecrystals 45 separated from the slurry are discharged, for example in apublic landfill site, and the aqueous mother liquor 46 is recycled tothe reaction chamber 31.

In an alternative form of the process according to the invention, it mayprove desirable to dilute the aqueous mother liquor 41 with additionalwater (not represented) to prevent sodium chloride from crystallizing inthe crystallizer 42.

In the operation of the process which has just been described, theamount of calcium chloride 32 introduced into the crystallizationchamber 31 is calculated according to the respective contents of sodiumsulphate and of potassium chloride in the brine 48, so that all thepotassium is precipitated in the form of glaserite in the crystallizer42 and so that the balance of sulphate ions is precipitated in the formof glauberite in the reaction chamber 31.

In the alternative implementation represented schematically in FIG. 2,the brine 24 (contaminated by potassium chloride, sulphate ions andsodium carbonate) is treated in a reaction chamber 50 with a sufficientamount of carbon dioxide 51 to decompose the sodium carbonate and tocrystallize sodium bicarbonate. An aqueous suspension 26 is collectedand is treated on a filter 52 to separate the sodium bicarbonatecrystals 53. The brine 27 collected from the filter 52 is subsequentlytreated as stated above, with reference to FIG. 1, first in the reactionchamber 25 with hydrochloric acid 47, to free it from the final tracesof sodium carbonate, and then in the degassing column 29, to remove theCO₂ generated in the chamber 25. This alternative implementation of theprocess according to the invention exhibits the advantage of recoveringsodium bicarbonate, which it is possible to exploit in the reactionchamber 7 (FIG. 1) in purifying the flue gas 6.

What is claimed is:
 1. Process for the production of sodium chloridecrystals from a sodium chloride brine contaminated by potassium chlorideand sulphate ions, according to which, in a first stage, a calciumcompound is added to the brine to precipitate calcium sulphate, whichisolated, and an aqueous solution is collected, in a second stage, theaqueous solution from the first stage is subjected to evaporation, tocrystallize sodium chloride, and sodium chloride crystals and an aqueousmother liquor are collected separately, and, in a third stage, theaqueous mother liquor from the second stage is subjected to cooling inorder to crystallize at least a portion of the potassium chloride,wherein the amount of calcium compound in the first stage and thecooling in the third stage are adjusted so that the sulphate ionsprecipitate in the form of glauberite in the first stage and ofglaserite in the third stage.
 2. Process according to claim 1, whereinsaid calcium compound comprises calcium chloride.
 3. Process accordingto claim 1, wherein said evaporation is carried out at a temperature of80 to 120° C. and the cooling is carried out at a temperature of 10 to70° C.
 4. Process according to claim 1, wherein before the second stage,sodium carbonate is added to the aqueous solution collected from thefirst stage to precipitate the residual calcium ions in the form ofcalcium carbonate.
 5. Process according to claim 1, wherein if the brineis contaminated by sodium carbonate, it is treated, before the firststage, with hydrochloric acid in order to decompose the said sodiumcarbonate.
 6. Process according to claim 1, wherein the brine comprises,per kg of dry matter, from 550 to 800 g of sodium chloride, from 50 to350 g of sodium sulphate and from 5 to 100 g of potassium chloride. 7.Process according to claim 6, wherein the brine comprises, per kg of drymatter, from 50 to 250 g of sodium carbonate.
 8. Process according toclaim 1, comprising the use, in the aqueous mother liquor, of an amountof water sufficient to prevent sodium chloride from crystallizing in thethird stage.
 9. Process according to claim 1 wherein the brine employedin the first stage is obtained from a gas contaminated with hydrogenchloride by adding a basic sodium compound to form a dust comprisingsodium chloride, sodium sulfate, potassium chloride and dispersing saiddust in water or an aqueous medium.
 10. Process according to claim 9,wherein said basic compound comprises sodium bicarbonate and in that thebrine comprises sodium carbonate and is treated, before the first stage,with carbon dioxide in order to crystallize sodium bicarbonate, which iscollected and recycled in the basic compound.